Naked Chocolate Peanut Butter Layer Cake

plus 2 tablespoons unsweetened cocoa powder*

and ¼ cups all-purpose flour

and ¼ cups sugar

and ¼ teaspoons baking soda

baking powder

salt

large eggs

plus 2 tablespoons warm water

plus 2 tablespoons buttermilk

safflower oil

pure vanilla extract

butter, softened (1½ sticks)

creamy peanut butter

powdered sugar

heavy cream, whipped to stiff peaks

dark chocolate, chopped

heavy cream

peanut butter cups to garnish (optional)